Armed separatists in eastern Ukraine have for a second day prevented international observers from reaching the wreckage of a Malaysian airliner that was shot down on July 17, killing all 298 people on board.

The Organization for Security and Cooperation in Europe (OSCE) said its team of monitors was only allowed to visit a part of the field strewn with debris from the airliner.

Alexander Hug, the chief of the OSCE special monitoring mission to Ukraine, said the team were allowed "to see a bit more" of the sprawling crash site.

Separatists on July 18 blocked the inspectors from visiting the crash site.

U.S. Secretary of State John Kerry told Russian Foreign Minister Sergei Lavrov that Washington was concerned over denial of "proper access" for international investigators and OSCE monitors.

According to the White House, Kerry also told Lavrov that Washington was "very concerned" over reports that the remains of victims and debris from the site of the crash have been removed or tampered with.

"This is unacceptable and an affront to all those who lost loved ones and to the dignity the victims deserve," Psaki said in a statement.

British Prime Minister David Cameron, writing in "The Sunday Times," said that if it were proven that Ukrainian separatists were behind the downing of the airliner, Russia would be to blame for having destabilized the country.

President Petro Poroshenko said in a statement that he would "not tolerate interference" with the work of international investigators.

Poroshenko also urged the United Nations on July 19 to label rebels fighting his forces in the eastern regions of Luhansk and Donetsk as belonging to "terrorist organizations."

Earlier on July 19, Ukraine's government accused separatists of trying to destroy evidence of an international crime "with the help of Russia."

Kyiv also said separatists moved 38 bodies to a morgue in Donetsk.

A rebel commander told the OSCE team on July 19 that separatists did not touch evidence and had not found the plane's "black box" flight data recordings.

Accusing Kyiv of the delay, he called on Moscow to help with the recovery of the bodies, which were starting to rot after two days in scorching summer heat.

Ukraine's counterintelligence chief, meanwhile, said he had "compelling evidence" that the plane was shot down by a Buk antiaircraft missile system operated by a crew of Russian citizens.

Ukraine's Security Council also said it had no information about any of the black boxes from the Malaysian airliner, and that the flight data boxes had not been handed over to them.

The UN Security Council has unanimously called for a "full, thorough, and independent international investigation" and urged all parties to grant investigators access to all parts of the sprawling crash area.

A German government spokesman said Chancellor Angela Merkel and Russian President Vladimir Putin have agreed that an independent, international commission led by the International Civil Aviation Organization (ICAO) should be granted swift access to the crash site.

In a telephone call on July 19, Merkel also urged Putin to use his influence with the separatists to reach a cease-fire in Ukraine.

Britain's Foreign Secretary Philip Hammond said "the world's eyes will be on Russia to see that she delivers on her obligations."

The office of the British prime minister said David Cameron and his Dutch counterpart, Mark Rutte, agreed on July 19 that the European Union will have to reevaluate its approach to Russia due to evidence indicating that Ukrainian pro-Russian separatists downed the Malaysian airliner.

Speaking in The Hague, Rutte said that -- in what he described as a "very intense" telephone conversation on July 19 -- he had told Putin that the Russian leader has one "last chance" to help recover the victims of the downed plane.

Rutte said he was shocked at pictures of "utterly disrespectful behaviour" toward the victims.

Interpol and Europol investigators were traveling to Ukraine on July 19 to help identify crash victims. A team of experts from Malaysia also arrived in Kyiv on July 19.

'Wake-Up Call'

On July 18, U.S. President Barack Obama said evidence indicates the plane was shot down by a surface-to-air missile that was launched from an area controlled by Russian-backed separatists.

Obama said the downing should be a "wake-up call for Europe and the world."

Obama accused Moscow of letting separatists bring weapons into eastern Ukraine from Russia and criticized the Kremlin for failing to stop the violence that made it possible to shoot down the plane.

In telephone communications and online posts, separatist fighters initially bragged about shooting down a plane in the area on July 17 before it became clear that a civilian passenger plane was targeted.

But leaders of the self-proclaimed "Donetsk People's Republic" now deny any involvement and claim a Ukrainian air force jet brought down the plane.

Russia's Defense Ministry has blamed Ukrainian ground forces, claiming that Russia's military picked up radar activity from a Ukrainian missile system south of Donetsk when the airliner was brought down.

But Australia’s Prime Minister Tony Abbott has lashed out at Russia for blaming Ukraine.

Abbott said Russia is "substantially to blame" for the problems in Ukraine -- and "the idea that Russia can somehow say that none of this has anything to do with them because it happened in Ukrainian airspace, frankly does not stand up to any serious scrutiny."